Defective bone remodeling is the pathophysiologic basis of most metabolic bone diseases, including postmenopausal and age-dependent osteoporosis. In adults, new bone formation follows an osteoclast mediated resorptive phase, and requires the synchronized activity of bone forming cells. During the tenure of this project, we have demonstrated that osteoblasts are interconnected in a "functional syncytium" by gap junctions formed primarily by connexin43 (Cx43). We have shown that targeted deletion of Cx43 in mice leads to developmental abnormalities of the skeleton, impaired osteoblast differentiation and reduced mineralization. We have also found that transcriptional regulation of genes pivotal to osteoblast differentiation and function, in particular osteocalcin (OC) is dependent on gap junctional communication, and have identified the -95 to -47 region of the OC promoter as a gap junction sensitive transcriptional unit. Further, we have biochemically purified heterogeneous nuclear ribonucleoprotein K (hnRNPK) as a putative gap junction sensitive transcription factor. The central hypothesis of this proposal is that the type of gap junctional communication provided by Cx43 is critical for osteoblast function and bone formation in the adult skeleton. The proposed experimentation will make use of genetically engineered mice and cell models of disrupted Cx43 expression, in part developed in our laboratory, to test the central hypothesis at 3 levels: organ, cellular and molecular. Accordingly, we propose to; 1: Determine skeletal maturation and response to osteoblast stimulation in mice with targeted deletion of the Cx43 gene in osteoblasts; 2: Determine the mechanisms by which lack of Cx43 alters osteoblast differentiation and function; 3: Identify gap junctional communication sensitive transcriptional factor(s) binding to the proximal region of the OC promoter. We anticipate that deletion of Cx43 in osteoblasts will result in achievement of inadequate peak bone mass, reduced anabolic response to intermittent PTH, impaired osteoblast differentiation and matrix production, and reduced transcriptional activity of hnRNPK or other gap junction sensitive transcription factors. These studies will disclose novel mechanisms by which the activity of bone forming cells is regulated by intercellular communication. Alterations of connexin expression and/or function by hormonal imbalances or aging may lead to osteoblast dysfunction and consequent demineralization.